1. Field of the Invention
The present invention relates to a switching power supply device.
2. Description of the Related Art
Switching DC stabilized power supply devices are used to, for example, convert commercial AC to DC. In this type of power supply device, a switching circuit converts DC power produced by rectifying and smoothing commercial AC power to AC power which is then fed to a primary winding of a transformer. The AC power induced in a secondary winding of the transformer is rectified and smoothed, so that DC power is outputted. Output voltage fluctuation is fed back to control the switching circuit, so that the output voltage is stabilized.
The power supply device is provided with a control power supply circuit to output a control voltage. The control voltage is a DC voltage to operate a control circuit for controlling the switching circuit. In order to generate a control voltage, the transformer has an auxiliary winding. The control power supply circuit converts an AC voltage induced in the auxiliary winding to a control voltage having a predetermined value.
The power supply device is so configured that a set value of the output voltage can be changed. For example, the power supply device switches the set value from a first value to a second value smaller than the first value when a load device to which the output power is supplied is turned from a normal mode to a sleep mode (power-saving mode). The power supply device switches the set value from the second value to the first value when the load device exits the sleep mode to enter the normal mode.
In response to the switching of the set value of the output voltage, the value of the AC voltage induced in the auxiliary winding, namely, the output level of the auxiliary winding, varies greatly. In order to operate the control circuit correctly, it is necessary to supply a control voltage having a value which is within the range of the rated voltage.
Conventional technologies have been proposed for supplying a control voltage having an appropriate value to the control circuit in Japanese Unexamined Patent Application Publication Nos. 2005-341709 and 2013-223280.
The former publication discloses a circuit configuration in which an auxiliary winding is provided so that an AC voltage equal to or greater than a necessary control voltage value is induced when the set value of the output voltage is low, and after the induced AC voltage is rectified, a 3-terminal regulator (dropper) is used for reduction to the control voltage.
The latter publication discloses the following: auxiliary windings different in numbers of turns are provided in a transformer. In accordance with a control signal for setting an output voltage, a driving voltage (control voltage) is generated based on any of AC voltages induced in the auxiliary windings.
In the meantime, a conventional technology has been proposed for reducing power consumption of a power supply device (Japanese Unexamined Patent Application Publication No. 2010-063293). The publication discloses stopping, in a standby state, supplying the electric current to a detection circuit for detecting an output voltage, and controlling a switching element so that a voltage developed in an auxiliary winding is constant instead of controlling the switching element based on a feedback signal from a secondary side.
As described in the technology of Japanese Unexamined Patent Application Publication No. 2005-341709, according to the circuit configuration in which the 3-terminal regulator is used to drop a voltage to generate a control voltage, power corresponding to the voltage drop is always consumed by the 3-terminal regulator. The technology thus has a problem of unnecessarily large loss of energy which causes the 3-terminal regulator to develop heat, in particular, when a set value of the output voltage is large in the power supply device.
The technology of Japanese Unexamined Patent Application Publication No. 2013-223280 has a problem that the transformer has to be large because of the plurality of auxiliary windings. Further, the plurality of rectifiers for rectifying the output of the auxiliary windings are needed, and they are used selectively depending on the set value of the output voltage, which lowers the efficiency of the circuit components.
The technology of Japanese Unexamined Patent Application Publication No. 2010-0632963 cannot solve the foregoing problems related to the control power supply circuit because the technology is directed to reduction in power consumption of the detection circuit.